A vial of AstraZeneca vaccine is seen at a mass COVID-19 vaccination clinic in Calgary on April 22, 2021. THE CANADIAN PRESS/Jeff McIntosh

A vial of AstraZeneca vaccine is seen at a mass COVID-19 vaccination clinic in Calgary on April 22, 2021. THE CANADIAN PRESS/Jeff McIntosh

10 weeks or 12? Expert says slight difference in AZ dosing interval likely negligible

Health Canada says the gap between shots can stretch 4 to 12 weeks

A push to administer thousands of COVID-19 vaccines before they expire has shortened the recommended dosing interval for some Ontarians, but one expert says that any difference in protection is likely negligible.

The province announced last week that the first cohort of Ontarians who received the Oxford-AstraZeneca shot in March were now eligible to schedule second jabs — marking 10 weeks from their first shot.

A recent study out of the United Kingdom found that lengthening AstraZeneca’s dosing interval to 12 weeks or more was more effective against COVID-19 than administering the doses six weeks apart.

The findings seemed to back Canada’s recommended strategy to delay second doses in order to administer more first doses. Health Canada says the gap between shots can stretch 4 to 12 weeks, while the National Advisory Committee on Immunization has recommended an extended interval of up to 16 weeks.

Dr. Allison McGeer, a professor in laboratory medicine and pathobiology at the University of Toronto, says taking a second vaccine dose at 10 weeks instead of 12 likely won’t impact its effectiveness.

“I can tell you that 12 weeks is better than four weeks, but I can’t tell you whether … 12 weeks is better than 10,” McGeer said during an Ontario Medical Association virtual press briefing on Wednesday.

“(But) personally, from my experience with vaccines, I’m with the province on this one.

“I do not think that your (immune) response is going to be different at 10 weeks than it is at 12 weeks.”

While some early AstraZeneca recipients may be wondering if they’re getting shortchanged by getting a second dose earlier, McGeer said vaccine schedules are typically “invented.”

“We decide what we think will work, and then we try it out and if it works we say, ‘Great,’ and we move on,” she said. “We don’t spend a lot of time saying: ‘Well, if we vaccinate you at 10 weeks or 12 weeks or 14 weeks or six weeks, is that a little bit better?’”

Ontario administered nearly one million doses of the AstraZeneca vaccine before halting first shots earlier this month due to what it said was an increased risk of blood clots.

The jab has been associated with vaccine-induced immune thrombotic thrombocytopenia (VITT), an extremely rare blood clotting condition that while treatable, can also be fatal.

The chances of acquiring VITT from a second shot of AstraZeneca are believed to be more rare than with the first dose, though McGeer said an exact estimate of risk is still unknown.

Those who received AstraZeneca for their first dose may be eligible for an mRNA vaccine from Pfizer-BioNTech or Moderna for their second shot, but the National Advisory Committee on Immunization is still waiting for efficacy data from a U.K. trial examining that mixed-dosing strategy.

McGeer said waiting for a potential mRNA dose could also put people at risk of contracting COVID-19 in the meantimebecause they’re not as fully protected from the virus.

“Now you’re balancing the ability to get a second dose of vaccine earlier, versus waiting for your second dose of an mRNA vaccine,” McGeer said.

READ MORE: NACI recommends same vaccine for 2nd dose; more data on mixing in mRNA vaccines coming

Ontario has about 45,000 doses of AstraZeneca expiring Monday,and pharmacies are aiming to dispense them within the next few days to avoid wastage. Another 10,000 doses of AstraZeneca expire next month, and the province has more than 300,000 doses in stock.

Dr. Zain Chagla, an infectious disease expert in Hamilton, says that with a steady, increased supply of mRNA vaccines coming in, it may be time for Canada to re-assess the amount of doses it needs right now.

He says some future shipments could be more useful in other countries that don’t have equitable access to the same inoculations.

“Does that mean we don’t vaccinate our population? No … but we do have to make difficult decisions about what the next three months of our vaccine plan looks like,” Chagla said. “Where do we set the cutoff to say: ‘OK, we’re not going to accept any more AstraZeneca into Canada … where is the cut off for Johnson & Johnson?

“Do we need this as part of our vaccine strategy or can we give that back to the world?”

Melissa Couto Zuber, The Canadian Press


Like us on Facebook and follow us on Twitter.

Coronavirusvaccines

Just Posted

The Trail Smoke Eaters will open the 2021 season on Oct. 8 against the Cranbrook Bucks in Cranbrook, and will have their home opener the next night against the same Bucks. Photo: Jack Murray
BC Hockey League announces 54-game schedule to begin in October

Trail Smoke Eaters open season with home-and-home series versus Cranbrook Bucks

Daryl Jolly, his wife Kerry Pagdin, their sons Cole Jolly (left) and Graeme Jolly, and their dogs Gracie and Clover. Photo: Submitted
Selkirk College arts chair diagnosed with lung cancer, family launches fund drive

Daryl Jolly co-founded the college’s digital arts program

TELUS is proposing to construct a 5G tower at Pople Park. Photo: Sheri Regnier
First 5G tower in Trail proposed for placement in popular park

TELUS has a consultation process open until June 28

The Kootenay International Junior Hockey League met for their AGM and announced a number of new initiatives, new awards and changes in their executive committee, as well as the starting date for the 2021-22 season. Paul Rodgers file.
KIJHL announces start dates for 2021-22 season

Season set to begin Oct. 1 with league still following all health guidelines

South Slocan’s Ti Loran is among the recipients of this year’s Neil Muth Memorial Scholarship. Photo: Submitted
Neil Muth Memorial Scholarships awarded to 4 students

Students in Creston, South Slocan and Revelstoke are sharing the honour

People watch a car burn during a riot following game 7 of the NHL Stanley Cup final in downtown Vancouver, B.C., in this June 15, 2011 photo. THE CANADIAN PRESS/Geoff Howe
10 years ago: Where were you during the 2011 Vancouver Stanley Cup Riots?

Smashed-in storefronts, looting, garbage can fires and overturned cars some of the damage remembered today

(Black Press Media file)
Dirty money: Canadian currency the most germ-filled in the world, survey suggests

Canadian plastic currency was found to contain 209 bacterial cultures

(pixabay file shot)
B.C. ombudsperson labels youth confinement in jail ‘unsafe,’ calls for changes

Review states a maximum of 22 hours for youth, aged 12 from to 17, to be placed in solitary

Grace (left), a caribou that was born in a maternal pen north of Revelstoke, is alive and well said the province. It appears she even has a calf. Maternity pens aim to increase caribou calf survival by protecting them from predation until they are older and less vulnerable. (Contributed)
For the first time in years, caribou numbers increasing near Revelstoke

North herd growing but south herd still concerning

Eleonore Alamillo-Laberge, 6, reads a book in Ottawa on Monday, June 12, 2017. THE CANADIAN PRESS/Sean Kilpatrick
Parents will need to fight ‘COVID learning slump’ over summer: B.C. literacy experts

Parents who play an active role in educating their children this summer can reverse the slump by nearly 80%, says Janet Mort

Kelowna General Hospital. (File photo)
COVID-19 outbreak at Kelowna General Hospital declared over

Three people tested positive for the virus — two patients and one staff — one of whom died

The border crossing on Highway 11 in Abbotsford heading south (file)
Western premiers call for clarity, timelines on international travel, reopening rules

Trudeau has called Thursday meeting, premiers say they expect to leave that meeting with a plan

The B.C. government’s vaccine booking website is busy processing second-dose appointments, with more than 76 per cent of adults having received a first dose. THE CANADIAN PRESS/Jonathan Hayward
B.C.’s COVID-19 infections, hospitalizations stable for Tuesday

108 new confirmed cases, 139 in hospital, 39 in intensive care

Most Read